1. Field of the Invention
The present invention relates to high density silicon nitride sintered bodies having excellent mechanical strength and oxidation resistance. More specifically, the invention relates to high density silicon nitride sintered bodies which particularly have excellent static fatigue characteristics and in which an intergranular phase is substantially crystallized into a diopside structure type crystals and apatite structure type crystals.
2. Related Art Statement
Since the silicon nitride sintered bodies are more excellent in terms of mechanical strength at high temperatures, thermal resistance, thermal shock resistance, and corrosion resistance than metallic materials, the former have been examined to be applied to high temperature structural members for which the latter can not be used, and uses of the former have been actively being developed.
Silicon nitride is a covalent substance, and therefore cannot be easily sintered through a solid phase. Thus, it is sintered through a liquid phase in which the silicon nitride is densified through production of a glass phase at a firing temperature under addition of Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, MgO, CeO.sub.2, SiO.sub.2, ZrO.sub.2, a rare earth oxide, AlN, or the like. For this reason, the silicon nitride sintered bodies thus obtained each contain a large amount of the glass phase produced in the grain boundaries during firing. Thus, when a use temperature is high, the intergranular glass phase is softened so that the mechanical strength and the oxidation resistance are lowered, while the characteristics of static fatigue due to creep deformation and creep rupture are deteriorated.
Under these circumstances, there have been studied methods of diminishing the glass phase through crystallizing the intergranular phase. For instance, Japanese patent application Laid-open No. 55-3,397 discloses silicon nitride sintered bodies which are fired with addition of Y.sub.2 O.sub.3 and SiO.sub.2 and contain a crystalline phase consisting of Y.sub.2 O.sub.3.2SiO.sub.2 and 10Y.sub.2 O.sub.3.9SiO.sub.2.Si.sub.3 N.sub.4 at grain boundaries. Japanese Patent application Laid-open No. 56-59,674 discloses silicon nitride sintered bodies which are fired with addition of Y.sub.2 O.sub.3 and contain xY.sub.2 O.sub.3.ySi.sub.3 N.sub.4 crystalline phase in grain boundaries. Further, Japanese patent application Laid-open No. 59-8,670 discloses silicon nitride sintered bodies in which an intergranular phase is a mellitite mineral phase represented by (Si, Mg, Y) (O, N). Japanese patent publication No. 58-50,944 discloses a method of crystallizing Y.sub.2 O.sub.3.Si.sub.2 N.sub.4 or Ce.sub.2 O.sub.3.Si.sub.3 N.sub.4 in an intergranular phase by reheating a silicon nitride sintered body added with Y.sub.2 O.sub.3 or CeO.sub.2. Furthermore, F.F. Lange "CERAMIC BULLETIN 62 (12) 1369-1374" shows that an intergranular phase is crystallized into Y.sub.5 (SiO.sub.4).sub.3 N crystals of an apatite structure. Each of the above silicon nitride sintered bodies having the intergranular crystalline phase has an improved high temperature strength.
However, since the grain boundaries are crystallized during a cooling step from a firing temperature or a reheating step in an inert atmosphere, the crystallized phase formed at the grain boundaries are not necessarily stable in an oxidative atmosphere as ordinary use conditions. Consequently, when they are used in the oxidative atmosphere, the crystalline phase varies and its volume locally changes, so that cracks are formed to greatly deteriorate mechanical strength and oxidation resistance. While the silicon nitride sintered bodies in which a majority part of grain boundaries are made of a crystalline phase is free from creep deformation and creep rupture because softening of the intergranular glass phase under a static loading does not occur at high temperatures, they are statically fatigued, without being deformed, due to a subcritical crack growth in which initial stage defects granually progress. Then, they are fractured after a given time period even under a stress lower than that corresponding to an instant fracture strength such as a bending resistive strength, etc. This static fatigue characteristics can be evaluated based on a stress dependency of a time in which the silicon nitride is fractured. The subscritical crack growth causing the static fatigue is considered to be influenced by thermal and chemical stabilities and a mechanical strength of the intergranular phase, a stress developed in the microstructure during cooling from the firing temperature owing to a difference in thermal expansion between the intergranular phase and the Si.sub.3 N.sub.4 crystalline particles, adhesion between the intergranular phase and the Si.sub.3 N.sub.4 crystalline particles, a quality and an amount of the residual glass phase, etc. The silicon nitride sintered bodies having the crystallized intergranular phase has a shortcoming that its reliability as a material of parts to be used for a long time period under a stress is lowered due to a static fatigue fracture caused by this subcritical crack growth.